Flotation systems are important unit operations in process engineering technology that were developed to separate particulate constituents from slurries. Flotation is a process whereby air is bubbled through a suspension of finely dispersed particles, and the hydrophobic particles are separated from the remaining slurry by attachment to the air bubbles. The air bubble/particle aggregate, formed by adhesion of the bubble to the hydrophobic particles, is generally less dense than the slurry, thus causing the aggregate to rise to the surface of the flotation vessel. Separation of the hydrophobic particles is therefore accomplished by separating the upper layer of the slurry which is in the form of a froth or foam, from the remaining liquid.
The fundamental step in froth flotation involves air bubble/particle contact for a sufficient time to allow the particle to rupture the air-liquid film and thus establish attachment. The total time required for this process is the sum of contact time and induction time, where contact time is dependent on bubble/particle motion and on the hydrodynamics of the system, whereas induction time is controlled by the surface chemistry properties of the bubble and particle.
However, flotation separation has certain limitations that render it inefficient in many applications. Particularly, in the past it has been thought that flotation is not very effective for the recovery of fine particles (less than 10 microns in diameter). This can be a serious limitation, especially in the separation of fine minerals. An explanation for this low recovery is that the particle's inertia is so small that particle penetration of the air-liquid film is inhibited, thus resulting in low rates of attachment to the bubbles. Furthermore, flotation has never been relied on as a process to effect separation of hydrocarbons in a slurry.
A further limitation of conventional flotation systems is that nominal retention times in the order of several minutes are required to achieve successful separation. However, it has been shown that air bubble/particle attachment is frequently in the order of milliseconds, therefore indicating that the rate of separation is mostly limited by bubble-to-particle collisions and/or transport rather than by other factors. As such, these long retention times severely limit plant capacity and require the construction of relatively large and expensive equipment.
Air-sparged hydrocyclones (hereinafter "ASH") were developed to overcome these two limitations of conventional flotation systems. Early systems such as disclosed in Russian Patent 692634 (Oct. 25, 1979) and in German Patent 1,175,621 (Aug. 13, 1964) were relied on to effect separation in a Centrifugal field by introducing air bubbles in the swirling stream. Refinements on this concept have been made such as exemplified in U.S. Pat. Nos. 4,279,743, 4,397,741, 4,399,027 and 4,744,890 which disclose certain improvements in ASH units. ASHs combine flotation separation principles with centrifugal forces to achieve successful separation of finer particles with retention times in the order of several seconds. A controlled high force field is established in the ASH by causing the slurry to flow in a swirling fashion, thereby increasing the inertia of the finer particles. Also, high density, small diameter air bubbles are forced through the slurry to increase collision rates with the particles. The net result is flotation rates with retention times approaching intrinsic bubble attachment times. This corresponds to a capacity that is at least 100 to 300 times the capacity of a conventional mechanical or column flotation unit.
In ASH flotation, fluid pressure energy is used to create rotational fluid motion (swirling motion). This is done by feeding the slurry tangentially through a conventional cyclone header into a cylindrical vessel. A swirl flow of a certain thickness is developed in the circumferential direction along the vessel wall, and is discharged through an annular opening created between the vessel wall and a pedestal located axially on the vessel's bottom.
Air is introduced into the ASH through the jacketed porous vessel walls, and is sheared into numerous small bubbles by the high velocity swirl flow of the slurry. Hydrophobic particles in the slurry collide with the air bubbles, attach to the bubbles, and are transported radially by the bubbles into a froth phase that forms in the cylindrical axis. The froth phase is supported and constrained by the pedestal at the bottom of the vessel, thus forcing the froth to move upward towards the vortex finder of the cyclone header, and to be discharged as an overflow product. The hydrophillic particles, on the other hand, generally remain in the slurry phase, and thus continue to move in a swirling direction along the porous vessel wall until they are discharged with the slurry phase through the annulus opening between the vessel wall and the pedestal.
It is important to note that the swirling motion of the slurry along the vessel wall forms a "swirl-layer" that is distinguishable from the forth phase at the center of the cylindrical vessel. One important characteristic of the swirl-layer is that it has a net axial velocity toward the underflow discharge annulus between the vessel wall and the froth pedestal. The thickness of the swirl-layer is generally 8% to 12% of the vessel radius, and it increases with increasing air flow rate and with axial distance from the cyclone header, being greatest at the underflow discharge annulus.
The size and motion features of the froth formed along the cylindrical vessel's axis are dependent on operating conditions and feed characteristics. Between the swirl-layer and the froth core, there exists a transition region for the slurry, where the net velocity in the axial direction is either zero, or in the same direction as the slurry phase. The latter condition exists where the froth core is relatively small, thus leaving a large gap between the swirl-layer and the froth core track is filled with slurry. The most desirable condition is when the transition region is minimal, that is when the froth core is large enough to leave little space between it and the swirl-layer.
A pressure drop is created in the froth core, between the froth pedestal and the vortex finder outlet located axially at the top of the vessel. This pressure drop is the force that actually drives the froth axially upwards. There are three factors that affect the pressure drop in the forth core:
1. restriction of the slurry flow to the underflow discharge annulus; PA1 2. restriction of the froth transport to the overflow vortex finder opening; and PA1 3. continuous supply of fresh froth to the froth core from the swirl-layer. PA1 1. effecting suspension and dispersion of small particles to prevent sedimentation and to permit contacting with air bubbles; PA1 2. influx of air, bubble formation, and bubble dispersion; PA1 3. conditions favouring particle bubble contact and attachment; PA1 4. a non-turbulent surface region for stable froth formation and removal; and PA1 5. in some cases sufficient mixing for further mineral reagent interaction. PA1 F.sub.m is the magnetic force PA1 V.sub.p is the volume PA1 J.sub.p is the magnetic polarization of the particle PA1 .gradient.B.sub.o is the gradient of the external magnetic field PA1 .mu..sub.o is the permeability of the medium. PA1 D is the demagnetizing factor of the particle, and is 0&lt;D&lt;1; and PA1 B.sub.o is the magnetic flux density. PA1 For para-magnetic particles, D&lt;&lt;1, therefore J.sub.p .congruent..chi.B.sub.o, and equation (1) becomes: EQU F.sub.m =V.sub.p .chi.B.sub.o .gradient.B.sub.o /.mu..sub.o (3) PA1 (i) the particles themselves (size, magnetic susceptibility, density); PA1 (ii) the retention time of separating forces acting on particles; PA1 (iii) the magnitude and geometry of the non-uniform magnetic field; and PA1 (iv) the geometry of magnetic and non-magnetic discharge posts. PA1 i) introducing a stream of the slurry into a cylindrical chamber having a cylindrical inner wall, the chamber being vertically oriented and closed at its lower end and open at its upper end, the stream being introduced near the first end at an incline and tangentially of the chamber to develop a spiral flow of the stream along the chamber inner wall toward the open end, PA1 ii) introducing the stream in sufficient volume and pressure to develop a vortex in the slurry which extends downwardly from the chamber upper end, PA1 iii) introducing air into the stream during at least a portion of its upward travel, the air being introduced to the stream through means located at the chamber inner wall and for developing the air bubbles which move into the stream, PA1 iv) the chamber being of a height sufficient to provide a residence time in the chamber which permits a separation of particles on their physical, magnetic and/or chemical properties with at least lighter hydrophobic particles combining with air bubbles and moving inwardly towards the vortex and at least heavier particles under influence of centrifugal forces of the spiral flow, moving outwardly towards the chamber inner wall, the stream developing into a whirlpool at the chamber upper end, PA1 v) directing the whirlpool stream outwardly at the open end into a catch basin surrounding the open end, the whirlpool stream swirling outwardly as the stream flows into the catch basin having a liquid level proximate the open end to permit the air bubbles to float toward a peripheral edge of the catch basin, PA1 vi) separating the floating air bubbles with lighter hydrophobic particles from the heavier particles by collecting outwardly floating air bubbles from an upper zone of the catch basin, while the heavier particles sink downwardly of the catch basin and removing the heavier particles from a lower zone of the catch basin to effect the separation. PA1 i) a cylindrical tube defining an interior cylindrical chamber with a cylindrical inner wall, and a closed lower end, PA1 ii) the inner wall having along at least a minor portion thereof and extending therearound, means for introducing gas bubbles into the inner chamber as a liquid slurry passes over the gas introducing means, PA1 iii) means for introducing a stream of slurry tangentially of and inclined relative to the inner wall, the stream introducing means being positioned in a lower zone of the chamber to direct a slurry stream in a spiral manner at the incline, PA1 iv) a catch basin surrounding an open upper end of the chamber to receive slurry overflowing the open upper end, PA1 v) the upper end having a smoothly curved edge portion to facilitate a smooth transition in flow of the slurry from a vertical direction to an outward direction as slurry overflows into the catch basin, PA1 vi) means for collecting froth generated in the slurry by bubbles introduced by the gas introducing means, the froth collecting means surrounding the catch basin, a weir being provided around the catch basin to define an overflow for froth floating outwardly of the catch basin, whereby froth overflowing the weir is collected in the froth collecting means, PA1 vii) the catch basin having an outlet in its lower portion to permit removal of sinking particles of liquid, PA1 viii) the froth collecting means having an outlet to permit removal of froth from the collecting means, PA1 ix) the catch basin outlet having means for controlling flow of liquid to maintain in the catch basin an acceptable height of liquid to permit froth to overflow the weir.
Factors 1 and 2 are in turn dependent on the particular application and can be adjusted during the operation. Factor 3 is dependent on air flow rate and on the hydrophobic properties of the particles, and their weight fraction in the feed slurry.
An immediate advantage of the ASH is the directed motion and intimate contact between the particles in the swirl-layer on the porous vessel wall and the freshly formed air bubbles. The high centrifugal force field developed by the swirling slurry imparts more inertia to the fine particles so that they can impact the bubble surface and attach to the bubbles. As a result, separation of fine particles is enhanced.
However, ASHs are relatively poor separators of coarser hydrophobic particles because the velocity of the swirling slurry imparts too high an inertia to these particles, thus preventing these particles from attaching to the air bubbles. As such, to achieve separation of these coarser particles, it is necessary that they exhibit relatively strong hydrophobicity so that the bubble/particle aggregate are stable under the prevailing hydrocyclone conditions. In cases where hydrophobicity is not strong enough, the system will exhibit some characteristics of a classification cyclone in that the coarse hydrophobic particles will be transported by the slurry to the underflow discharge annulus, while the finer particles will have a tendency to be transported into the froth core and out through the overflow vortex finder.
Studies have shown that the separation efficiency for a number of mineral particles falls as particle diameters increase above 100 microns. However, other studies show that the upper particle size limit is strongly affected by the hydrophobicity of the particle (as discussed above), and thus can be extended beyond 100 microns. For coal particles, testing shows that separations of particles above 100 to 400 microns drops significantly with increasing slurry pressure.
Therefore, an important addition to the art would occur if a method and apparatus is developed that can effectively separate particles of sizes beyond the present range of particle sizes. Also, a significant improvement would occur if increased slurry pressure (therefore increased feed flow rates) can be used while maintaining efficient separation. An important development in the method and apparatus is described in applicant's published application WO 91/15302 published Oct. 17, 1991 with surprising degrees of particle separation involving unique application of separation techniques in an ASH. As a guide in further understanding the principles of separation in the new ASH of applicant, one may refer to the published PCT application. However, as an overview the following principles are discussed to provide a better understanding of the benefits provided by applicant's discovery set out in this application.